Harvey Swadlow: Brain state and the awake thalamocortical network
Talk by Harvey Swadlow, Department of Psychology, University of Connecticut. Given to the Redwood Center for Theoretical Neuroscience at UC Berkeley on March 4, 2010.
Producer Redwood Center for Theoretical NeuroscienceAudio/Visual sound, color
Awake subjects may shift between alert and non-alert brain states hundreds of times each day, but the effects of this state transition on sensory processing have been difficult to capture. In order to maintain control of the sensory stimulus, most visual studies in the awake state have been done in when subjects are alert and attentive. Studies of the awake primate visual system, for example generally require subjects to fixate on a small, stationary stimulus. If this is not done, the gaze will shift and stimulus placement will vary in retinal coordinates. In the rabbit, the eyes remain relatively stable when the head is fixed, even when subjects shift between alert and non alert EEG states. This allows analyses of receptive field and other response properties before and after shifts between brain states.
This talk will have four components: (1) the first describes the behavior of corresponding classes of cortical neurons in four sensory and motor cortical regions (V1, S1, S2 and motor cortex) in awake rabbits. In these studies, a great diversity was seen in the spontaneous spiking activity, receptive field properties and axonal properties of five classes of cortical neurons in each of these cortical areas. The neuronal classes include four types of cortical projection neurons (callosal, ipsilateral corticocortical, descending corticofugal neurons of layer 5, corticothalamic neurons of layer 6) as well as putative fast spike interneurons. It was shown that the properties of these different cell classes within each cortical area differ greatly. Whereas some are predominantly silent and unresponsive, others are highly active and responsive. However, the same classes of neurons found in different cortical regions behave with remarkable similarity. Notably, the differences among cell classes within cortical regions, and the common features of corresponding classes across regions was maintained across shifts in brain state. (2) In the second part of the talk we descend to the thalamus and examine the dramatic changes that occur in LGNd neurons within a single second of the shift from alert to non-alert states. These changes occur in temporal tuning, contrast response functions, spontaneous activity and in "bursting". (3) We then follow the flow of visual information from the thalamocortical (TC) neuron to the TC synapse, and ask how the "efficacy" and the dynamics of this synapse is influenced by the shift between alert and non-alert states. There is good reason to suspect that TC synaptic efficacy would be affected by state changes. However, we did not find this to be the case. We did document a very strong effect of preceding interspike interval on the strength of the postsynaptic response generated by impulses of single TC neurons (because TC synapses are depressing, TC neurons are active, and long preceding interspike intervals allow recovery from synaptic depression). However, when preceding intervals were equivalent, no state effect was observed. (4) Finally, we cross the TC synapse to the input layer of the visual cortex (layer 4) and examine the effects of state-shifts on the spontaneous activity and visual response properties of putative spiny cells with "simple" receptive fields and on putative fast-spike interneurons of this layer. If time permits we will also describe the effects of state shifts on the response properties of corticotectal neurons, and on backpropagation along the apical dendrite of these cells.
Supported by the U.S. National Eye Institute and National Science Foundation.